(1) Field of the Invention
The present invention relates to method and apparatus for detecting the ignition time point of an engine, more particularly to a method for detecting the ignition or the ignition time point of an engine from the output signal from a flame sensor mounted at the combustion chamber of the engine and an apparatus for the same.
(2) Description of the Prior Art
Recently, in the field of art concerning Diesel engines, there has been proposed a system for detecting the ignition of fuel injected and for precisely controlling the fuel timing and fuel injection amount by a feedback loop in accordance with the fuel ignition state. In such a control system mentioned above, a flame sensor for detecting the ignition state of the fuel is used which is, for instance, shown in FIG. 1 as a flame sensor 1. The flame sensor 1 comprises a cylindrical housing 2 within which a light guide 3 made of a heat-resistant, light-transmissive material such as quartz glass is fitted. The light guide 3 has a detection end 4 which is slightly projected out of the cylindrical housing 2.
As shown in FIG. 2, the flame sensor 1 is mounted in such a manner that the detection end 4 projects inside the combustion chamber 6 of a Diesel engine 5 and the fuel thus injected from a fuel injection nozzle 7 can reach near detection end 4. The burning flame light detected by the flame sensor 1 mounted in the combustion chamber 6 is transmitted through a light transmitting means such as an optical fiber 8 to a photoelectric converting means not shown, such as photodiode, phototransistor or photocell where the burning flame light is converted into an electrical signal i.e. voltage signal as shown in FIG. 3. The combustion state or condition of the fuel in the combustion chamber 6, however, varies dependent to the actual running condition of the engine, i.e. the engine speed, the magnitude of the fuel injection amount, etc.
For instance, if the engine load which corresponds to the ratio between the engine speed and the fuel injection amount is high, the waveform of the voltage signal detected by the ignition sensor or flame sensor becomes large after a photoelectric conversion as shown in FIG. 3 (a), while if the engine load is low, the waveform of the voltage signal becomes small as shown in FIG. 3 (b). However, the ignition start time point in which the voltage signal rises does not change at all, even if the load is changed as marked by as show the arrow shown in the figure.
The time point of the rise of the signal mentioned above is indicated by the rise of the voltage signal slightly above 0 volt. However, since the signal in the vicinity of 0 volts is superimposed with noise, the threshold level of the detected signal Vt is determined so as to avoid false indication of the voltage signal due to noise. Accordingly, when the waveform (the peak value) of the signal differs from signal to signal as shown in FIG. 3 (a) and (b), the discrepancy among the signal detecting timing or time point will occur by the time .DELTA.t, thus making the accurate detection of the ignition timing impossible.